The soil pipe for the toilet in my bathroom is also connected to the tub and sink drains directly below the toilet. The sink drain leaks where it connects with the 4" lead drain pipe. The lead drain pipe then connects to a cast iron elbow. I want to replace the lead portion with PVC and also replace the toilet flange with a PVC flange. I have been told that connecting PVC to Cast iron can be tricky. Is this a do-it-yourself job, or should I leave this one to a professional?

I highly recommend leaving it to a professional, but if you want to try it all you need to connect pvc to cast iron is a 4" no hub addapter and a 4" no hub band. That's the only way to connect pvc to cast iron.